1. Field of the Invention
The present invention relates generally to a charge/discharge control circuit for controlling the charge and discharge of a secondary battery and to a rechargeable or charging-type power-supply unit with a secondary battery that includes the charge/discharge control circuit.
2. Related Background Art
A power-supply unit as shown in a circuit block diagram in FIG. 4 is known as a conventional charging-type power-supply unit including a secondary battery. Such a configuration is disclosed in, for instance, Japanese Patent Application Laid-open No. Hei 4-75430 xe2x80x9cA Charging-Type Power-Supply Unitxe2x80x9d. That is, a secondary battery 101 is connected to an external terminal xe2x88x92V0 or +V0 through a switching circuit 102. In addition, a charge/discharge control circuit 110 is connected to the secondary battery 101 in parallel. This charge/discharge control circuit 110 has a function of detecting the Voltage of the secondary battery 101.
When the secondary battery 101 is either in an overcharged state (the state where the battery has a higher voltage than a predetermined voltage: hereinafter referred to as an xe2x80x9covercharge protective statexe2x80x9d) or in an overdischarged state (the state where the battery has a lower voltage than a predetermined voltage: hereinafter referred to as an xe2x80x9coverdischarge protective statexe2x80x9d), a signal is outputted from the charge/discharge control circuit 110 so that the switching circuit 102 is turned off. Further, the switching circuit 102 is set to be turned off to stop discharge when the external terminal +V0 reaches a certain voltage, so that a current flowing through the switching circuit 102 can be limited. In other words, discharge can be stopped (overcurrent can be controlled) when an excessive current flows. Hereinafter, this state is referred to as an xe2x80x9covercurrent protective statexe2x80x9d. The charge/discharge control circuit serves to protect the battery from such states.
It is necessary for this charge/discharge control circuit 110 to output a control signal for turning the switching circuit 102 on when a load 103 is disconnected from between the terminals +V0 and xe2x88x92V0 after the overcurrent protective state is realized.
FIG. 2 is a block diagram showing a charging-type power-supply unit including a conventional charge/discharge control circuit. When the load 103 is connected and the Voltage of an overcurrent detecting terminal exceeds that of a reference Voltage source 122, the charge/discharge control circuit detects overcurrent. When the overcurrent is detected, an overcurrent detecting comparator 120 outputs a high-level Voltage which serves as an overcurrent detecting signal. The output signal from this overcurrent detecting comparator 120 serves as an input signal to an internal control circuit 210. The internal control circuit outputs a signal for turning the switching circuit 102 off. At this time, the internal control circuit 210 outputs a signal to a gate of an N-channel transistor 201 so that the overcurrent detecting terminal can be connected to a VSS terminal on a lower Voltage side of the secondary battery 101 through a resistor 123. Afterward, when the load 103 is disconnected from terminals +V0 and xe2x88x92V0, the voltage of the overcurrent detecting terminal drops to the same electric potential as the voltage of the VSS terminal since the overcurrent detecting terminal is connected to the VSS terminal through the resistor 123. In this case, the output signal from the overcurrent detecting comparator 120 is reversed, and the internal control circuit 210 outputs a signal for turning the switching circuit 102 on to cancel the overcurrent state.
In the case of employing the above-mentioned circuit configuration, however, there has been a problem when a charger 104 is connected, the secondary battery 101 is brought into an overcharged state, and thus the switching circuit 102 is turned off. In the overcharged state, the switching circuit 102 is turned off, so that the charging of the secondary battery 101 is prohibited. In the circuit configuration shown in FIG. 2, however, although the switching circuit 102 is turned off, a parasitic diode 202 is present in the N-channel transistor 201. Therefore, there has been a problem in that a charging current may flow into the secondary battery 101 through the resistor 123 and the parasitic diode 202.
In order to solve the above problem, in the present invention, it is detected that a charger is connected, and then a resistor is disconnected from an overcurrent detecting terminal, so that a charging current is prevented from flowing into a secondary battery through a parasitic diode even when a switching circuit is turned off and thus the charger is connected.